The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
An exhaust gas recirculation (EGR) device is a device that reduces nitrogen oxides (NOx) in exhaust gas discharged after combustion in an engine. The EGR device recirculates a part of inert exhaust gas into an intake system of the engine in order to reduce generation of NOx by decreasing a maximum temperature at the time of combustion in the engine.
Meanwhile, in a case in which the exhaust gas is recirculated into the intake system of the engine, ignition ability of a gaseous mixture or output of the engine may deteriorate. Therefore, in order to efficiently reduce NOx and ensure drivability of the engine, an EGR rate of the EGR device is controlled in accordance with a driving state of the vehicle.
In order to improve an effect of increasing the EGR rate in an EGR operation section, a method of retarding an exhaust cam for opening and closing an exhaust valve is used. In the case of retarding the exhaust valve, there is a valve overlap period for which an intake valve and the exhaust valve are maintained in an opened state at the same time.
In a case in which there is the valve overlap period, exhaust gas is completely discharged, such that volumetric efficiency of a cylinder is improved, and a cooling effect is improved by reducing a cause of premature ignition.
In contrast, there is a concern in that fuel consumption is increased because the gaseous mixture is discharged together with the exhaust gas for the valve overlap period. Particularly, in a low-speed driving range, because of the valve overlap period, a shock is caused by combustion instability, or a back-fire phenomenon occurs in which combustion occurs through an intake valve.